Flat resistance for blower control unit for automobile air conditioner and blower control unit using the same

ABSTRACT

A flat resistance for a blower control unit of an automobile air conditioner, and a blower control unit using the same. The flat resistance includes: a porcelain enameled metallic substrate including a flat head portion, having an edge and one surface, and parallel terminal supporting portions projecting outwardly from the edge of the head; a resistance circuit printed on the one surface of the head portion, the resistance circuit including a plurality of resistances electrically connected in series; a temperature fuse, interposed in the resistance circuit, for being tripped to break the resistance circuit when the porcelain enameled metallic substrate becomes overheated; and terminals, printed on both the head portion and the terminal supporting portions, each terminal being connected to one end of a corresponding one of the resistances.

This is a division of application Ser. No. 07/417,571, filed Oct. 5,1989 now U.S. Pat. No. 5,000,662.

BACKGROUND OF THE INVENTION

The present invention relates to a flat resistance for a blower controlunit of an automobile air conditioner, and particularly relates to aflat resistance having a plurality of resistances connected in series onan electrically insulating substrate for controlling the revolution ofthe blower. The present invention further relates to such a blowercontrol unit using the flat resistance.

The blower of the automobile air conditioner has a fan installed withina fan scroll The fan is coupled to a fan motor, of which revolution iscontrolled by means of the blower control unit. The blower control unit,is provided with a variable resistance circuit including a plurality ofresistances connected in series. By varying the resistance of theresistance circuit in a stepwise manner, voltage applied to the fanmotor is varied to control the revolution of the fan motor so that themotor is selectively operated in one of a high speed mode, middle speedmode and low speed mode, for example

In one of typical examples of the conventional blower control unit, eachresistance of the resistance circuit is in the shape of a coil and isplaced within an air duct of the automobile air conditioner,particularly in the vicinity of a blowout opening of an intake unitthereof, for rapidly removing Joule's heat generated from theresistances by the air stream passing through the air duct. The coilresistances have some resistance to the air stream in the air duct witha rather large pressure loss, and large noises can be produced by theair stream. Thus, the resistances do not meet recent strong requirementsof large flow rate of the air conditioner and low noise in the interiorof the motor car.

To reduce such drawbacks, a flat resistance in which a plurality ofresistances are printed in series on a flat ceramic substrate has beenproposed (for example, in Japanese Patent Laid-open (unexamined)Publication 62-88610). The ceramic substrate is inferior in strength andhas relatively low heat radiation, which necessitates heat radiatingfins. This prior art flat resistance hence requires a considerablethickness, which provides a tendency to cause large pressure loss andnoise to be produced when the flow rate of the blower is large.

Accordingly, it is an object of the present invention to provide a flatresistance for a blower control unit of an automobile air conditionerand a blower control unit using the flat resistance, which enables bothpressure loss of the forced air and noises produced by the flatresistance to be considerably reduced.

SUMMARY OF THE INVENTION

With this and other objects in view, one aspect of the present inventionis directed to a flat resistance for a blower control unit of anautomobile air conditioner, including: a porcelain enameled metallicsubstrate including a flat head portion, having an edge and one surface,and parallel terminal supporting portions projecting outwardly from theedge of the head; a resistance circuit printed on the one surface of thehead portion, the resistance circuit including a plurality ofresistances electrically connected in a series-parallel arrangement, atemperature fuse, interposed in the resistance circuit, for beingtripped to break the resistance circuit when the porcelain enameledmetallic substrate becomes overheated; and terminal means, printed onboth the head portion and the terminal supporting portions, terminalmeans including terminals each connected to one end of a correspondingresistance.

Another aspect of the present invention is directed to a blower controlunit of an automobile air conditioner, using the flat resistance abovedefined. The terminal means comprises a high speed mode terminaldirectly connected to one end of the resistance circuit, and the blowerincludes a fan. The blower control unit includes: a fan scroll includinga bottom wall, and a spiral side wall joined to the bottom wall tosurround the fan; mounting means for perpendicularly mounting thesubstrate to the bottom wall of the fan scroll at a position of amaximum flow rate within the fan scroll to be parallel with air fromthee blower; a fan motor electric circuit including the fan motor andadapted to connect to the high speed mode terminal; and switching means,interposed between the fan motor electric circuit and the terminals, forselectively connecting the terminals to the fan motor electric circuit.The side wall of the fan scroll has a nose portion and an innercircumferential wall portion facing the nose portion, and the substrateis arranged at an angle within ±10° to the inner circumferential wallportion of the side wall.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example withreference to the drawings in which:

FIG. 1 is a circuit diagram of an electric circuit of a blower controlunit according to the present invention;

FIG. 2 is an enlarged sectional view taken along the line I--I in FIG.1;

FIG. 3 is a perspective view of a flat resistance in FIG. 1;

FIG. 4 is a horizontal sectional view, in a reduced scale, illustratingan automobile air conditioner having the flat resistance of FIG. 1incorporated into it;

FIG. 5 is a sectional view taken along the line V--V in FIG. 4;

FIG. 6 is a perspective view of the fan scroll in FIG. 4;

FIG. 7 is an enlarged horizontal section around the flat resistance ofFIG. 4;

FIG. 8 is a graph showing the result of Example 1 and illustrating therelationship between the inclination angle of the electricallyinsulating substrate and static pressure;

FIG. 9 is a front view illustrating a modified form of the flatresistance in FIG. 1 with the supporting frame sectioned;

FIG. 10 is a front view of the insulating substrate in FIG. 9;

FIG. 11 is a sectional view taken along the line XI--XI in FIG. 9;

FIG. 12 is a sectional view taken along the line XII--XII in FIG. 9

FIGS. 13 and 14 are perspective views showing modified forms of theinsulating substrate in FIG. 9, respectively;

FIG. 15 is an enlarged sectional view of the boss of FIG. 14;

FIG. 16 is a front view of a modified insulating substrate of FIGS. 1-3;

FIG. 17 is a front view of a further modified form of the insulatingsubstrate in FIG. 16;

FIG. 18 is a front view illustrating the insulating substrate of FIG.17, the insulating substrate being connected to a feeder connector orcoupler;

FIG. 19 is a front view of a modified form of the insulating substratein FIG. 17;

FIG. 20 is a front view of a modified insulating substrate in FIG. 1;

FIG. 21 is an illustration of a temperature distribution of theinsulating substrate, having no temperature fuse, in FIG. 20 whenvoltage is applied between the terminals T1 and T2;

FIG. 22 is an illustration of a temperature distribution of theinsulating substrate, having no temperature fuse, in FIG. 20 whenvoltage is applied between the terminals T1 and T3;

FIG. 23 is an illustration of a temperature distribution of theinsulating substrate, having no temperature fuse, in FIG. 20 whenvoltage is applied between the terminals T1 and T4;

FIG. 24 is an enlarged fragmentary sectional view showing a modifiedform of the temperature fuse in FIGS. 1 and 2;

FIGS. 25 and 26 are perspective views illustrating still modified formsof the temperature fuse in FIG. 24, respectively;

FIGS. 27A-27C are front views showing various modified forms of shapesof the opposing open ends of the resistance circuit for the temperaturefuse, respectively;

FIG. 28 is a modified form of the temperature fuse in FIG. 24;

FIGS. 29A-29C are sectional views illustrating how the temperature fuseof FIG. 28 is tripped;

FIGS. 30-32 are front views illustrating still modified forms of thetemperature fuse in FIG. 24;

FIG. 33 is an enlarged vertical sectional view showing a modified formof a fitting end of one of the terminal supporting portions of theinsulating substrate of FIG. 1;

FIGS. 34-36 are illustrations as to how to manufacture the insulatingsubstrate with terminal fitting ends of FIG. 33;

FIG. 37 is an enlarged vertical sectional view of a fitting end of aterminal supporting portion of an ordinary insulating substrate;

FIGS. 38 and 39 are vertical sectional views, reduced in scale,illustrating modified forms of the fitting end portion of FIG. 33;

FIG. 40 is a vertical cross-sectional view showing an ordinary terminalfitting end portion having a conductor circuit provided to it; and

FIG. 41 is an enlarged vertical cross-sectional view showing a modifiedform of the terminal fitting end portion of FIG. 38, having anelectrically conductive coating formed over it.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a flat resistance FR according to the presentinvention is electrically connected at a high speed terminal T1(hereinafter referred to as HI terminal) of the terminals T thereof to amotor M, which drives a fan f of a blower of an automobile airconditioner. The terminals T of the flat resistance FR are connected bya fan switch SW across a battery B of the automobile to the motor M witha current fuse F3 interposed between the battery B and the fan motor M.The terminals T include the HI terminal, a middle-high speed terminal T2(hereinafter as MH terminal), a middle-low speed terminal T3(hereinafter as ML terminal) and a low speed terminal T4 (hereinafter asLo terminal).

The fan switch SW selects one of the terminals T by sliding a contact Kto control the revolution of the fan motor M in a step-wise manner.

As shown in FIG. 3, the flat resistance FR has an electricallyinsulating substrate 3, which is a porcelain enameled metallicsubstrate, and a supporting frame 5 fitted around the substrate 3 insuch a manner that a head 7 of the substrate 3 is projected upwards fromthe supporting frame 5. The supporting frame 5 has a socket portion 9extending downwards and surrounding the terminals T when the substrate 3is fit into the supporting frame 5. A feeder connector or coupler 10(FIGS. 5 and 9) is fitted into the socket portion 9 to electricallyconnect to the terminals T. The substrate 13 is produced by coating aheat-resistant insulating layer 13, such as alkali-free crystallizedporcelain enamel, over a metal core 11 such as a decarburized enamelingsteel plate, a stainless steel plate and a copper plate. A resistancecircuit R is printed on one surface of the head 7 of the substrate 3,the resistance circuit R having a plurality of, three in thisembodiment, resistances R1-R3 connected in a series-parallelarrangement. The resistance circuit R is covered with conventional heatresistant insulating layer 15 for protection.

The resistance circuit R is formed by printing a predetermined patternof a resistance paste for the resistances R1-R3 and baking it on thesubstrate 3. The opposite ends of each resistance R1, R2, R3 areelectrically connected to corresponding terminals T1-T4. The resistanceR1 which bridges between the HI and MH terminals T1, T2 is provided witha first temperature fuse F1 which when overheating of the resistancecircuit R takes place due to blocked motor M for example, melts to breakthe circuit R, thereby deenergizing the motor M. The temperature fuse F1is electrically connected to a broken or open portion of the resistanceR1 near the MH terminal T2 by soldering.

The resistances R1-R3 are reduced in cross-section in the describedorder. Thus, resistor R3 may be formed the shortest among the resistorsR1-R3 for an equal resistance. With this, flexibility both in printingspace and arrangement of the printing pattern is improved and the totalsize of the flat resistance FR may be miniaturized. The resistance R3 isprovided with a second temperature fuse F2 as in the resistance R1. Thetemperature fuse F2 improves response of the resistance circuit R.

In this embodiment, as illustrated in FIGS. 4-6, the flat resistance FRis arranged in a fan scroll 19 of an intake unit 17 of the automobileair conditioner. The supporting frame 5 is fastened by machine screws toa bottom wall 19A of the fan scroll 19 in such a manner that the headportion 7 of the substrate 3 projects into an air passage 21 within thefan scroll 19. The head portion 7 is preferably arranged in the vicinityof the fan f within the fan scroll 19 for cooling with the forced airfrom the fan f. In usual automobile air conditioners, the intake unit 17introduces air from the inside or outside of the automobile into anevaporator 23 of a cooler unit 25 for cooling, and then the cooled airis fed to a heater core (not shown) of a heating unit 27, which has anair distribution door or an air deflecting rib (both members not shown).In FIGS. 4 and 5, reference numeral 31 designates a dash panel of theautomobile, 33 a side panel, 35 a supply air switching box, 37 an intakedoor, 39 a cowl box, 41 a motor cooling duct and 43 a nose portion ofthe fan scroll 19.

The flow rate of the fan f is changed at the air distribution door orthe air deflecting rib, and hence it is preferable to install the head 7of the substrate 3 in the vicinity of the blowout opening 29 of the fanscroll 19, where the flow rate is the maximum, and as close to the fan fas possible. However, the head portion 7 which generates heat duringoperation should not be arranged excessively close to the fan f or thewalls of the fan scroll 19 when they are made of a synthetic resin.

The substrate 3 is preferably arranged in parallel with the air streamin the air passage 21 so that noise is not produced during operation ofthe fan f. However, such a parallel arrangement of the substrate 3 isnot a sufficient condition. When the substrate 3 suspends from theceiling wall 19B of the fan scroll 11, heat which is generated from thehead portion 7 thereof can give a damage to the ceiling wall 19B. whenthe substrate 3 is horizontally mounted to a side wall 19C of the fanscroll 19 to project into the air passage 21, the dash panel 31 or theside panel 33 which are located in the vicinity of the substrate 3 canhinder attachment and replacement of the substrate. Thus, it ispreferable to mount the substrate 3 to the bottom wall 19A at apredetermined interval 1 from the side wall 19C, which interval gives noheat damage to both the side wall 19C and the fan f.

To enhance the heat dissipation, the head portion 7 of the flatresistance FR is preferably installed at a position, where the flow rateof the forced air is the maximum, in this embodiment a position in thevicinity of the blowout opening 29 of the fan scroll 19.

From the point of the air resistance, the head portion 7 is arrangedsubstantially in parallel with the stream of the forced air. To do sothe head portion 7 is theoretically mounted with the plane thereofplaced in parallel with a tangent to the side wall 19c of the fan scroll19 or a tangent to the fan f if it is a centrifugal fan. For thisreason, in this embodiment, the head portion 7 is placed in parallelwith a flat portion A of the side wall 19c of the fan scroll 19 since itcannot be substantially parallel with the air stream, flowing spirallyin the fan scroll 19, at the other portions of the fan scroll 19. Theflat portion A faces a nose portion 43 of the fan scroll 19 and extendslinearly. The inclination angle θ of the head portion 8 to the flatportion A is usually about -30° to about +20° for a flow rate of 5 m³/min, and about -20° to about +20°, preferably about -10° to about +10°for a flow rate of 8 m³ /min. In view of low noise , caused by the airstream, as well as the low air resistance, an inclination angle θ of 0°to -10° provides the optimum result at the flow rate of 8 m³ /min.

FIGS. 9 to 12 illustrate a modified form of the flat resistance of FIGS.1-3. The modified flat resistance includes a base portion 6 integrallyformed with the head portion 7 in parallel with the general plane of thelatter. The base portion 6 has four parallel terminal supportingportions 51A, 51B, 51C, 51D projecting from it. Two outer terminalsupporting portions 51A, 51D have each a locking projection projectingperpendicularly outwards from outer edges thereof. The supporting frame5 includes a substantially rectangular head plate portion 54 and asocket portion 9 integrally formed with the lower surface of the headplate portion 54 to extend downwards. The head plate portion 54 has alongitudinal base portion receiving slot 50 formed in it to have abottom wall 55 as a ceiling wall of the socket portion 9. The ceilingwall 55 is provided with four longitudinal through slots 57A, 57B, 57B,57A for passing the terminals T1, T2, T4, T3 into a socket cavity 52,respectively. The through slots 57A, 57A through which the outerterminal supporting portions 51A, 51D pass have a lager length than theother through slots 57B, 57B and are also wider than the latter as shownin FIGS. 11 and 12. The width of the through slots 57B, 57B is slightlysmaller than the width of the inner terminal supporting portions 51B,51C for resiliently holding them by snugly fitting. The wall 56 of thebase portion receiving slot 50 has a resilient tongue catch 61integrally formed with it in the vicinity of the each through slot 57A,57A to project diagonally downwards. Each catch 61 is provided at itsdistal end 62 with a locking shoulder 63 with which the correspondinglocking projection 53 is to engage. Each locking projection 53 and thecorresponding catch 61 constitutes a detachment preventing mechanism 67.

In fitting the base portion 6 of the porcelain enameled metallicsubstrate 3 to the supporting frame 5, each of the locking projections53 comes into contact with and resiliently deforms the correspondingcatch 61 in a direction to be away from it as the terminal supportingportion 51A or 51D passes through the corresponding through slot 57A.Then each locking projection 53 drops in the locking shoulder 63 of thecorresponding catch 61, so that the catch 61 return to its originalposition, shown in FIG. 11, to thereby lock the locking projection 53with the distal end of each of the terminals T1-T4 placed in positionwithin the socket cavity 52. The supporting frame 5 of the flatresistance FR thus assembled is fitted at its head portion 54 into anattachment opening 60 (FIG. 11) of the bottom wall 19A of the fan scroll19 with the head portion 7 of the substrate 3 projected into the fanscroll 19. In this position, the flat resistance FR is fastened at itsflanges 58, 58 to the bottom wall 19a with machine screws not shown.

In inserting a feeder connector 10 into the socket 9, the insertingforce of the connector 10 is exerted on the porcelain enameled metallicsubstrate 3 through the terminal supporting portions 51A-51D. In thisembodiment, the locking projections 53, 53 engage with correspondingresilient tongue catches 61 which are slantingly mounted to the headportion 54 of the supporting frame, and hence most of the insertingforce of the feeder connector 10 is absorbed by the catches 61, 61.Thus, the substrate 3 is steadily supported by the supporting frame 5and is prevented from coming off the supporting frame 5 by fitting thefeeder connector 10 in the socket portion 9.

The detachment preventing mechanism of the porcelain enameled metallicsubstrate 3 may include various modifications. In FIG. 13, the substrate3 is provided with a pair of outermost projections 68, 68 in parallelwith the terminal supporting portions 51A-51D. A locking projection 69as the first locking member is struck out of each outermost projections68, 68. The locking projections 69, 69 are adapted to engage withcorresponding catches 61, 61 of the supporting frame 5.

Alternatively, as the first locking member a pair of locking projections70, 70 in the shape of a boss may be provided close to the lower edge ofthe head portion 7 of the substrate 3 as illustrated in FIGS. 14 and 15.

In place of the locking shoulder 63, as the second locking member, ofeach catch 61, a projection, hole or nail may be provided to the catchto engage with the first locking member 53, 69 or 70.

A modified form of the insulated substrate of FIGS. 1-3 is illustratedin FIG. 16, in which reference numeral 71 indicates a fitting portion tofit into a feeder connector 10 (FIG. 9), the fitting portion 71 beingconnected to the resistance circuit R through a corresponding electricconductor circuit 73. The conductor circuits 73 serve to preventoverheating of corresponding fitting portions 71 by controlling heattransfer from the resistance circuit R to those fitting portions. Eachconductor circuit 73 typically has a sheet resistivity of aboutmΩ/□(mΩ/square unit) or less and preferably has as small a resistance aspossible. The necessary length of the conductor circuits 73 depends onthe sheet resistivity. Preferably, the conductor circuits 73 have alength at least about 5 mm when they have a sheet resistivity about 10mΩ/□ or less. When the sheet resistivity of the conductor circuits 73exceeds about 10 mΩ /□, an effect of controlling heat transfer to thefitting portions becomes rather small.

FIG. 18 is an illustration as to how to connect a porcelain enameledmetallic substrate of FIG. 17 to a feeder connector 10. This substratewill be described in connection with Example 5 later.

FIG. 19 shows a modified form of the substrate 3 of FIG. 17. In thismodified substrate, part of the resistance circuit R is replaced by alow resistance conductor. More specifically, a low resistance conductoris printed and baked at a part of the resistance R1 at 75. In addition,resistances R1, R2, R3 may be reduced by printing and baking conductorcircuits 77 to connect to corresponding conductor circuits 73 which areprovided for preventing overheating of terminals T1-T4. Resistances R1,R2, R3 may be increased by reducing corresponding conductor circuits 73to a minimum length at which the connected terminals may not becomeoverheated. According to this modification, the resistance of theresistance circuit may be easily and promptly varied without changingthe pattern, the width and the length thereof, that is, withoutconsiderably changing heat balance of the whole substrate. In theprinting and baking process of the resistance circuit, the change ofresistances R1-R3 may be performed by refabricating only patterns of theconductor circuits 77, for example Ag patterns.

The maximum temperature zone in overheating of the resistance circuitdepends upon the pattern of the resistance circuit itself. Thus, theprovision of a conductor circuit 75, which generates little heat whencurrent flows through it, provides an effect such that the substratefunctions as if the permissible maximum temperature of the substratewere raised.

A modified form of the porcelain enameled metallic substrate of FIG. 1is shown in FIG. 20, in which a resistance circuit R is a thin film orthick film which is formed on the substrate by plating or sputtering.Temperature fuses F1 and F2 are furnished to the resistance circuit R ata position or in the vicinity of the position where the substrate heatedto the highest temperature when the resistance circuit R is overheated.Although this modification may be applied to a resistance circuit havinga single resistor, it is very effective for such a resistance circuit asthe resistance circuit R, in which a desired resistance or resistancesare selected from a plurality of resistances by switching terminals. Inthe resistance circuit, the temperature distribution pattern is rathercomplicated and it is hard to determine the highest temperature zone.

In this modified form, the temperature fuses F1 and F2 are mounted tohighest temperature zones of the substrate when Tm≈Tmax, where Tmax isthe permissible maximum temperature of the substrate and Tm is themelting temperature of the temperature fuses F1 and F2. When Tm<Tmax,then the temperature fuses F1 and F2 are arranged at zones where thetemperature becomes higher than Tm when the substrate is heated to thepermissible maximum temperature Tmax.

To determine the position of the first temperature fuse F1, a resistancecircuit of the porcelain enameled metallic substrate 3, without anytemperature fuses, is connected to a power source to cause overheatingfor a short time and when the substrate 3 reached to the permissiblemaximum temperature Tmax, a temperature distribution of the substrate 3is observed by a thermoviewer or the like instrument. According to theobservation of the substrate, the first and the second temperature fusesF1 and F2 are located at determined positions. With such an arrangement,the fuses F1 and F2 are tripped to break the resistance circuit R whenor before the substrate is heated to the permissible maximum temperatureTmax by overheating of the resistance circuit R. When temperature fusesF1 and F2 are mounted on respective positions determined by means of thethermoviewer, the temperature distribution can slightly change since thesubstrate having the fuses generates little heat at the fuse positionsas compared to the original substrate. Therefore, the substrate with thefuses is tested as to whether or not the highest temperature of thesubstrate exceeds the permissible maximum temperature Tmax when thefuses are tripped. If it exceeds the permissible maximum temperatureTmax, the positions of the fuses must be changed.

It is preferable to set the difference between the permissible maximumtemperature Tmax and the melting temperature of the temperature fuses F1and F2 as small as possible since excessively large difference intemperature makes it difficult to raise temperatures of the temperaturefuses F1 and F2 to or above the melting temperature Tm when thesubstrate reaches the permissible maximum temperature Tmax. Morespecifically, the substrate is practically placed in an air stream forcooling, and hence changes in temperature distribution can be caused bythe flow rate and condition of air flow impinged upon the substrate.Thus, the provision of the temperature fuses at positions having such anexcessively large difference in temperature may cause the fuses not tomelt to break the resistance circuit when the substrate becomesexcessively heated. Preferably, the temperature difference is about 50°C. or smaller.

It is possible to manufacture a porcelain enameled metallic substratehaving a temperature fuse or fuses without preparing a substrate havingno fuse. The highest temperature of the substrate must not exceed thepermissible maximum temperature thereof when the fuse or fuses aretripped.

Provision of a temperature fuse to a resistor, for example resistanceR1, commonly used in various speed mode reduces the necessary number ofthe temperature fuses.

In the flat resistance of FIG. 20, a cream solder of a Sn-Pb eutecticsolder (melting temperature of 183° C.) is printed on broken portions ofresistance R1 and R3 and then melted by heating to form the temperaturefuses F1 and F2 to bridge respective broken portions. The porcelainenameled metallic substrate 3 has a permissible maximum temperature Tmaxof 200° C. and the positions of the temperature fuses F1 and F2 aredetermined by the method above stated.

The temperature fuse F1 serves as a fuse for both the middle high speedmode resistance circuit (R1) and the middle low speed mode resistancecircuit (R1 +R2). The temperature fuse F2 is a fuse for the low speedmode resistance circuit (R1+R2+R3).

FIG. 24 illustrates a modified form of the temperature fuses F1 and F2of FIGS. 1 and 2. The modified temperature fuse F is formed by printinga conventional solder paste on a substrate with a screen mesh or a metalmask and then by heating it to melt. The fuse F preferably has a smallthickness of about 0.1 to about 1.0 mm as compared to usual fuses madeof a solder rod or a solder thread. The solder paste used in thismodified fuse F may contain Pb and Sn as major components and inaddition silver, indium, antimony and bismuth.

When a screen mesh is used for printing the temperature fuse F, a screenof about 40 to about 80 meshes is preferably used. The thickness of theprinted fuse may be about 0.1 mm or larger, and thus, the thickness ofthe emulsion of the fuse material is about 100 to about 400 μm.Preferably, the metal mask made of a stainless steel has a thickness ofabout 200 to about 300 μm. The thickness of the solder fuse F may be setto about 0.1 to about 1.0 mm by adjusting the pressure and speed of theprinting and the hardness and the angle of the squeegee. Preferably, themetal mask made of a stainless steel has a thickness of about 200 toabout 300 μm. With a 200 to 300 μm thick metal mask, a fuse paste havinga thickness 1.0 mm or larger may be printed. However, excessively thickfuse paste causes the temperature fuse to form into a non-flat shapehaving a round upper surface after it is melted in manufacturing of thefuse, and hence a print thickness about 1.0 mm or smaller is preferable.In addition, the fuse material having a thickness larger than about 1.0mm can raise a problem in that the molten fuse material can move out ofa predetermined position to be printed during manufacturing of thetemperature fuse. When the thickness of the fuse material smaller thanabout 0.1 mm is used, shrinkage thereof which is produced duringreflowing can cause the printed pattern of the fuse to be partiallydamaged.

The production of about 0.1-1.0 mm thick temperature fuse by printingand reflowing enables highly accurate thickness adjustment thereof,which provides an accurate amount to the fuse. Thus, the operationtemperature of the fuse can be set to a narrow range.

The thick film temperature fuse enhances the space efficiency of theporcelain enameled metallic substrate and reduces the possibility of thesubstrate being damaged due to breaking of a stack of substrates duringpackaging or transportation.

FIG. 25 illustrates a modified form of the fuse in FIG. 24. In thismodification, the thickness t of the fuse F' is in a range of about 0.1mm to about 1.0 mm. The resistance circuit 80 has a cut off portion 81having opposing ends 83, 83 of which length L and width W are set twiceor more than twice as large as the thickness t of the fuse F'. The gap Gbetween the opposing ends 83, 83 is also twice or more than twice thethickness t. When such a fuse F' is melted by heating the substrate andcoagulated by surface tension, molten fuse F' is separated and isattracted to opposing ends 83, 83 since the areas of the opposing ends83, 83 and the gap G are sufficiently large for the amount of the moltenfuse F'. Thus, the resistance circuit 80 is positively broken.

When the porcelain enameled metallic substrate is to be installed withan inclination to a horizontal plane, in addition to the conditionsabove stated, a lower end 83A is, as shown in FIG. 26, preferably largein width and length as compared to an upper end 83B. A fuse F" is formedto cover these ends 83A and 83B. With such a fuse F", a molten fusewhich moves downwards is held by the lower end 83A, thus positivelybreaking the circuit without leaving any fuse between the ends 83A and83B.

The flux which is contained in the solder cream for the fuses F', F"remains on the surface of the fuses and is left without removing. Theflux prevents oxidation of the surfaces of the fuses F', F" andfacilitates coagulating of molten fuse. The fuses F', F" hence improvesdurability and reliability of operation.

The opposing ends 83, 83 may have various shapes shown in FIG. 27A-27C,provided they are made of a material having a good wettability to fusesF', F". The opposite ends 83, 83 may be wider or narrower than theresistance circuit 80.

A modified form of the fuse in FIGS. 1 and 2 is illustrated in FIG. 28,in which the modified fuse F1 includes a solder fuse 91 and silver fusebases 93, 93 made of a rectangular silver film piece which has a goodwettability to the solder fuse 91 in a molten state as compared to asilver-palladium alloy which is a base material of the resistance R1.The open portion 81 which breaks the resistance circuit R is formed bycutting off part of a printed pattern of the resistance R1. The heatresistant layer 13 or a through hole, if formed, is exposed through theopen portion 81 before the solder fuse 91 covers the open portion 81.The resistance R1 has opposing ends 83, 83, to which a silver materialis applied and baked to form a pair of silver fuse bases 93, 93,respectively. The silver fuse bases 93, 93 are formed to attract asolder fuse 91, thereby breaking the resistance circuit R with highresponsibility. Thus, fuse bases of good wettability to the molten fuse91 must be attached to respective end 83, 83. Other suitable materialhaving such a property may be used for the fuse bases 93 instead ofsilver. After forming of the silver fuse bases 93, 93, a solder fuse 91is bridged between them.

In operation, the temperature fuse F1, F2 melts when heat due tooverheating of the resistance circuit R is transferred to it (FIG. 29B).The molten fuse is attracted to the silver fuse bases 93, 93, so that itis pulled in the opposite directions. This results in rapid separationof the molten fuse (FIG. 29C) and hence the resistance circuit R isopened, thereby stopping power supply to the fan motor M.

The resistance R3 may be provided with a fuse F2 having similar fusebases as in the fuse F1 for improving responsibility thereof to enhancesafety of the substrate.

In FIG. 30, only one end 83 of the resistance R1 or R3 is provided withone silver fuse base 93. Even the single silver fuse base 93 achievessubstantially the same effect as the two silver fuse bases 93, 93 inFIG. 28.

Another modified form of the fuse F1, F2 is illustrated in FIG. 31, inwhich one fuse base 95 has a width larger than the other one 97. Withthe difference in attracting force of the molten fuse between the fusebases 95 and 97, responsibility of the fuse F1, F2 is improved.

In the fuses F1, F2 in FIGS. 28-31, the silver fuse bases 93, 95, 97 maybe coated over the open ends.

The fuse bases 93, 95, 97 are made of silver which is a substance havinga good wettability to the resistance circuit R made of asilver-palladium alloy. The substance used for the fuse bases 93, 95, 97is selected in view of both the interface tension between the moltenfuse and the resistance circuit R and that between the molten fuse andthe fuse bases. The substance of the fuse bases hence depends on thesubstances of both the resistance circuit R and the fuse F1, F2 andthus, various substances may be used for the fuse bases.

FIG. 32 shows a modified form of the fuses F1 and F2 of FIG. 1. In themodified fuse, open ends 83, 83 are formed by the screen printingmethod, in which a screen with 100 meshes or more, or a screen using athick mesh wire, having a diameter 0.15 mm for example, may be used forproviding a finely corrugated surface to the open ends 83, 83, thecorrugated surface having, for example, a pitch 0.01-0.2 mm and a depth5-20 μm. In this modification, the molten fuse is attracted to open ends83, 83 by means of capillary phenomenon of the corrugated surfaces ofthe ends 83, 83 produces. Such finely roughened surfaces of the ends 83,83 may be formed by subjecting the ends which have been formed byordinary screen printing methods to blasting or hair line processing.

FIG. 33 shows a modified form of the fitting portion 71 of each terminalsupporting portion 51 of the porcelain enameled metallic substrate 3 ofFIG. 1. In this modified form, the metal core 11 of the fitting portion71 is tapered thicknesswise toward the distal end 105 thereof forfacilitating inserting the fitting portion 71 into a feeder connector,thereby reducing damages to the conductor circuits and the metallicsubstrate. Preferably, the metal core 11 is usually about 0.5 to about0.7 mm thick is tapered over a distance b of about 1.0 to about 3.0 mm,preferably about 1.0 to about 1.5 mm so that the distal end 105 has athickness about 0.10 to about 0.30 mm. With a thickness a smaller thanabout 0.1 mm, the distal end 105 is liable to be broken off since itbecomes thinner during the acid cleaning as the pretreatment. When thethickness of the distal end 105 exceeds 0.3 mm, the difference inthickness between the distal end and the proximal portion of theterminal supporting portion 51 of the core 11 becomes rather small, andhence the swelling of the porcelain enamel layer 13 can not besufficiently prevented for relatively thin metal cores, thus increasingthe inserting force of the corresponding fitting portion into a feederconnection. The porcelain enamel layer 13 is thickest at a positionabout 0.6 cm away from the distal end 105 and hence the length b of thetapered portion is preferably in the range of about 1.0 to about 1.5 mm.When the length b is beyond about 3.0 mm, the angle of the taper becomesconsiderably small for a relatively thin metal core, and thus there is apossibility of forming the swelling of the porcelain enamel layer 13.

The tapering of the distal end portions 103 is preferably carried out atthe same stage as the punching of the metal core 11 from a blank bymeans of a punching die. This processing is performed by firstlypunching out an unnecessary portion, indicated by the dot-and-dash linein FIG. 34, of the blank to form distal ends of terminal supportingportions 51. Then, the distal ends, indicated by the dot-and-dash linein FIG. 35, are depressed thicknesswise to provide tapered distal ends103. In this event, burrs are produced to the distal ends 103. Finally,the metal core 11 is punched out by removing unnecessary portion 105together with the burrs. The metal core 11 thus prepared undergoes acidclearing, coating with a porcelain enamel, and then baking to producethe porcelain enameled metallic substrate 3 with tapered distal ends 103of the terminal fitting portions 71.

A further modification of the fitting portion 71 of FIG. 33 isillustrated in FIG. 38, in which an about 8-15 μm thick conductorcircuit 109 which constitutes part of the terminal T is formed on anupper surface 104A of the porcelain enamel layer 13 of each fittingportion 71 in vicinity to the distal end 107. The conductor circuit 109has a small friction coefficient to a feeder connector as compared tothe porcelain enamel layer 13 and hence reduces the inserting force toinsert each fitting portion 71 into a feeding connector.

Another conductor circuit 109' may be formed by printing a conductorpaste for the circuit on the lower surface 104B of each terminalsupporting portion 71. The conductor circuits 109' are provided to coverthe porcelain enamel layers 13 of corresponding fitting portions 71 onlyfor reducing friction to a feeder connector and hence the thicknessthereof may be about 8 μm.

A modified form of the fitting portion 71 of FIG. 38 is illustrated inFIG. 41, in which it is coated with a lubricating layer 111 made of anelectrically conductive lubricating agent which enables sufficientlyelectrical connection between the conductor circuit 109 and the feederconnector 10. As the conductive lubricating agent, use may be made of aconductive lubricating agent containing graphite, for example acolloidal graphite dispersed a liquid vehicle. Such a conductivelubricating agent may be coated over the fitting portion 71 by splayingor dipping. The conductive lubricating agent preferably has propertiessuch that the coating thereof cannot be easily removed by rubbing duringtransportation of the flat resistance and is capable of keeping thelubricating performance during long term use. With the lubricating layer111, oxidation and change in color of the conductor circuit 109 areprevented. Usual Ag conductor circuits without any lubricating layer 111change their color due to sulfurization.

EXAMPLE 1

A flat resistance FR having a head portion 7, which is 50 mm high, 75 cmwide and 2 mm thick, was arranged in a fan scroll 17 as illustrated inFIGS. 4-7. The spacing l from the flat portion A was 50 mm. The fan fwith a diameter 120 mm and a height 70 mm was rotated at a high speed toprovide a flow rate 8 m³ /min and at a normal speed to provide a flowrate of about 5 m³ /min. Under these conditions, the static pressuredownstream of the head portion 7 was determined when an inclinationangle θ which was formed between the head portion 7 and the flat portionA of the side wall 19c was varied to examine the air resistance of thehead portion 7.

The results are given in Table 1 and plotted in FIG. 8, from which it isclear that at the flow rate of 8 m³ /min, the inclination angle θ withina range of -10° to +10° provided a fairly high static pressure of about40 mmAq or larger. The static pressures measured at a flow rate of 5 m³/min are plotted by the broken line in FIG. 8, from which it is notedthat the static pressures were 50 mmAq or more. With the flow rate of 5m³ /min, no significant problem in air stream noise was raised at anyinclination angle θ.

                  TABLE 1                                                         ______________________________________                                        Inclina- Static Pressure                                                                              Static Pressure                                       tion θ                                                                           (mmAq) at 8 m.sup.3 /min                                                                     (mmAq) at 5 m/.sup.3 /min                             ______________________________________                                        no flat  45.0           58.5                                                  resistance                                                                    -30°                                                                            29.0           50.5                                                  -20°                                                                            37.5           56.0                                                  -10°                                                                            43.0           56.5                                                    0°                                                                            44.5           55.5                                                  +10°                                                                            45.5           48.5                                                  +20°                                                                            38.5           42.5                                                  +30°                                                                            12.5           30.5                                                  ______________________________________                                    

EXAMPLE 2 and COMPARATIVE TEST 1

Five 50 mm×60 mm porcelain enameled metallic substrates 3 as in FIG. 16were prepared as samples 1-5. The resistance circuit R of each substratehad a resistance 2Ω and was made of an Ag-Pd thick film having a sheetresistivity 45 mΩ/□. The fitting portions 71 and the conductor circuits73 of each substrate were made of an Ag thick film having a sheetresistivity 4 mΩ/cm².

Each of the substrates 3 was mounted within an air duct of a blower forthe interior of an automobile, and then current of 10 A was applied topass the resistance circuit, using metallic connectors. The temperatureof the fitting portions 71 of the terminals of each substrate wasmeasured using a thermoviewer when the highest temperature of thesubstrate reached to 135° C. The results are given in Table 2 in whichtemperatures of fitting portions 71 were indicated for lengths ofconductor circuit 73. The comparative test was conducted in the sameconditions except that no conductor circuit 73 was provided.

Table 2 indicates that the longer the conductor circuit, the lower thetemperature of the fitting portions 71. The temperature of the sampleNo. 5 having a length 4.5 mm raised to 60° C. which is the permissiblemaximum temperature of usual synthetic resin feeder connector: Thefitting portion of the sample of the comparative test was heated to 110°C. and it was not possible to use any feeding connector made ofconventional synthetic resins with low thermal resistance.

                  TABLE 2                                                         ______________________________________                                        Sample    Length of conductor                                                                          Temperature of fitting                               No.       circuit (mm)   portion (°C.)                                 ______________________________________                                        1         5              55                                                   2         10             44                                                   3         15             39                                                   4         20             35                                                   5         4.5            60                                                   Comparative                                                                             0              110                                                  Test                                                                          ______________________________________                                    

EXAMPLE 3

Porcelain enameled metallic substrates with four terminals were preparedas illustrated in FIG. 17, each substrate having a resistance circuit Rwith resistance R1-R3 having sheet resistivity 45 mΩ/□. In eachsubstrate, a high speed mode resistance circuit Ra or R1 betweenterminals T1 and T2, a middle low speed mode resistance circuit Rc orR1+R2 between terminals T1 and T3, and a low speed mode resistancecircuit Rb or R1+R2+R3 between terminals T1 and T4 were 0.5Ω, 1.5Ω,3.5Ω, respectively. The resistance circuits Ra-Rc were electricallyconnected to fitting portions 71 of the terminals through conductorcircuits 73A-73D which had a low sheet resistivity of 10 mΩ/58 .

Each of the substrate was attached to the air passage of the blower fanused in Example 2, and then the overheating preventing effect of theconductor circuits was tested for various lengths of conductor circuits.In these tests, 8 A, 6.5 A and 4 A currents were applied to resistancecircuits Ra, Rb and Rc respectively, and temperatures of the fittingportions 71 of terminals were measured by a thermoviewer when a highesttemperature portion of each substrate was heated to 157° C.

The results are given in Tables 3-5, which indicates that desiredoverheating preventing effects of the terminals were provided byselecting appropriately the length of the conductor circuits.

                  TABLE 3                                                         ______________________________________                                        Current 8A in the high speed mode resistance circuit Ra                       Length of Conduc-        Temperature of                                       tor Circuit (mm)         Terminal (°C.)                                73A      73B             T1    T2                                             ______________________________________                                        4        4               77    78                                             6        6               58    59                                             8        8               50    49                                             10       10              42    41                                             ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Current 6.5A in the middle high speed mode resistance circuit Rb              Length of Conduc-        Temperature of                                       tor Circuit (mm)         Terminal (°C.)                                73A      73B             T1    T2                                             ______________________________________                                        8        3               44    75                                             8        4               44    60                                             8        6               44    56                                             8        8               44    42                                             ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                        Current 4A in the low speed mode resistance circuit Rc                        Length of Conduc-        Temperature of                                       tor Circuit (mm)         Terminal (°C.)                                73A      73C             T1    T4                                             ______________________________________                                        8        3               40    70                                             8        4               40    58                                             8        6               40    45                                             8        8               40    40                                             ______________________________________                                    

EXAMPLE 4

A 50 mm×60 mm porcelain enameled metallic substrate was prepared and hadthe same resistance circuit as shown in FIG. 19 except the conductorcircuit 75 was not provided. The total resistance R1+R2+R3 was 2Ω andthe resistance circuit R was an Ag-Pd thick film circuit having sheetresistivity of 45 mΩ/□. The conductors 73 and the terminals T1-T4 wereAg thick film circuits having a sheet resistivity 4 mΩ/cm². Conductiveportions were added at 77 to corresponding conductor circuits 73 byprinting and baking the same Ag thick film circuits as the conductorcircuits 73. With this the total resistance R1+R2+R3 reduced from 2Ω to1.74 Ω. In this example, the width, the length and other shapes of thepattern of the resistance circuit were not changed.

EXAMPLE 5

The same porcelain enameled metallic substrate as in Example 4 was used,and an Ag thick film circuit was printed and baked at 75 in FIG. 19. Thethick film circuit 75 was made of the same Ag thick film as theterminals T1-T4 and the conductors 73. The finished resistance circuit Rhad a resistance 1.9 Ω. In both Examples 4 and 5, the printing andbaking of the Ag thick film circuit of the conductors 75 and 77 wereperformed simultaneously with the forming of the conductors 73.

EXAMPLE 6

A porcelain enameled metallic substrate 3 without any temperature fuse(that is, without any broken portions at positions where temperaturefuses F1 and F2 are mounted in FIG. 20) was manufactured. Thetemperature distribution of the substrate 3 was determined when thehighest temperature thereof reached 200° C. with a locked motor M of ablower. A temperature distribution produced by applying voltage to themiddle high speed mode resistance circuit or between terminals T1 and T2is shown in FIG. 21, another temperature distribution in which voltagewas applied to the middle low speed mode resistance circuit or betweenterminals T1 and T3 is given in FIG. 22, and a still anotherdistribution given by applying voltage to the low speed mode resistancecircuit or between terminals T1 and T4 is shown in FIG. 23. From theseresults, it was determined that two temperature fuses F1 and F2 having amelting temperature 183° C. were to be located at positions indicated bythe broken lines in FIGS. 21 to 23. In every operation mode of the motorM, one of the temperature fuses F1 and F2 was to be placed at positionswhere temperatures were above 183° C. in overheating of the substrate.In such a manner, the substrate 3 in FIG. 20 was fabricated.

                  TABLE 6                                                         ______________________________________                                        Terminals between which                                                                         Highest temperature                                         voltage is applied                                                                              of substrate (°C.)                                   ______________________________________                                        T1 and T2         186                                                         T1 and T3         195                                                         T1 and T4         187                                                         ______________________________________                                    

An examination was made as to how the temperature fuses F1 and F2 of thesubstrate 3 functioned with the motor locked. One of the fuses F1 and F2melted to break the resistance circuit when the substrate 3 reachedhighest temperatures, given in Table 6, below 200° C.

EXAMPLE 7

Porcelain enameled metallic substrates were prepared, each having twotemperature fuses A and B respectively provided to a low resistancecircuit and a high resistance circuit. The temperature fuses were formedin the manner described below so that they were actuated at an operatingtemperature of 230° C. A soldering paste containing 40 weight % of Snand 60 weight % of Pb was used for the temperature fuses. In everysubstrate, the temperature fuse A at the low resistance circuit and thetemperature fuse B at the high resistance circuit of each substrate were5 mm×6.5 mm and 5 mm×3 mm in area, respectively. A pattern of eachtemperature fuse was printed by means of a metal mask. The amount ofeach temperature fuse was set not to move out of the pattern when thefuse melted. Samples 1-3, 3-6, 7-9 and 10-12 were set to have thicknessof about 390 μm, 250 μm, 200 μm and 130 μm, respectively. Thetemperature fuses were tested by applying DC current of 8A and 4A to thelow resistance circuit and the high resistance circuit, respectively.

The outcomes of the test are given in Table 7, from which it would beclear that about 380-400 μm thick temperature fuse A and about 175-210μm thick temperature fuse B provided operating temperatures of about200°-230° C.

                  TABLE 7                                                         ______________________________________                                                             Thickness Oper. tem.                                     No.     Fuse         of fuse (μ)                                                                          (°C.)                                   ______________________________________                                        1       A            380       222                                            2       A            390       220                                            3       A            400       230                                            4       A            255       202                                            5       A            240       205                                            6       A            240       190                                            7       B            200       205                                            8       B            175       198                                            9       B            210       210                                            10      B            130       185                                            11      B            130       190                                            12      B            120       185                                            ______________________________________                                    

COMPARATIVE TEST 2

Porcelain enameled metallic substrates having temperature fuses A' andB', which were equal in area to temperature fuses A and B, respectively,were prepared. The fuses A' and B' were formed of a wire solder havingthe same components and compounding ratio as the soldering paste ofExample 6. The temperature fuses A' and B' of each substrate underwentthe same test as in Example 6.

The results of the test are provided in Table 8, form which it wasconfirmed that temperature fuses made of the soldering wire largelydiffered in amount. It was hence hard to set the operation temperaturethereof to within a practical range.

                  TABLE 8                                                         ______________________________________                                                             Amount of Oper. temp                                     No.     Fuse         fuse (mg) (°C.)                                   ______________________________________                                        13      A            230       263                                            14      A            220       226                                            15      A            135       267                                            16      A            160       240                                            17      A            190       249                                            18      B             48       300                                            19      B             65       234                                            20      B             47       239                                            21      B             53       260                                            22      B             60       245                                            ______________________________________                                    

EXAMPLE 8

Insulating substrates having temperature fuses A and B were prepared,each fuse having an amount within the appropriate range determined inExample 7 so that it was tripped at a predetermined operatingtemperature of 200° to 230° C. The temperature fuses A and B were testedas to whether or not they were actuated at predetermined as totemperatures. The results are given in Table 9A.

                  TABLE 9A                                                        ______________________________________                                                             Thickness Oper.temp.                                     No.     Fuse         of fuse (μ)                                                                          (°C.)                                   ______________________________________                                        23      A            400       228                                            24      A            370       210                                            25      A            390       205                                            26      A            410       227                                            27      A            410       225                                            28      A            400       215                                            29      A            410       231                                            30      B            385       225                                            31      A            420       226                                            32      A            410       212                                            33      A            370       198                                            34      A            420       229                                            35      B            190       200                                            36      B            190       206                                            37      B            180       220                                            38      B            220       220                                            39      B            200       202                                            40      B            190       210                                            41      B            220       235                                            42      B            200       220                                            43      B            200       217                                            44      B            225       230                                            45      B            210       220                                            46      B            175       211                                            ______________________________________                                    

EXAMPLE 9

A resistance circuit 80 made of a Ag-Pd thick film and having a width 2mm was formed on a porcelain enameled metallic substrate 3 as in FIG.25. The resistance circuit 80 was cut to form opposing ends 83, 83 witha 2 mm gap G. A solder cream (Sn-Pb eutectic solder: melting temperature183° C.) was applied by means of a metal mask 0.3 mm thick in arectangular shape over the opposite ends 83, 83 and the gap G. Theopposite ends 85, 85 of the solder cream had a length of 2 mm, so thatthe total length of the solder cream was 6 mm with the opposite ends 85,85 overlapped over the opposing ends of the resistance circuit 80. Then,the solder cream was melted to form a fuse F' having a thickness 0.2 mm.The fuse F' thus prepared was not washed, so that the flux was left overthe surface thereof. Such porcelain enameled metallic substrates wereprepared in the number of 100, each substrate having a single fuse F'.

The substrates were heated at a rate of temperature rise of 10° C./min.It was noted that the fuses F' partly began to melt at 183° C., and thatall the fuses F' were melted in 10 seconds from the beginning of themelting, so that molten fuse F' moved towards the opposing ends 83, 83of the resistance circuits 80, thereby positively breaking resistancecircuits 80.

EXAMPLE 10

A resistance circuit 80 having a width W2 1.0 mm was formed on aporcelain enameled metallic substrate 3 as in FIG. 26 in the same manneras in Example 9. A first fuse base 83A 2.0 mm wide (W1) and 2.0 mm long(L1) was connected to one end 83 of the resistance circuit 80 whereas asecond fuse base 83B 1.0 mm wide (W2) and 1.0 mm long (L2) was formed tojoin to the other end 83, with a gap G 1.0 mm from the first fuse base83A. The same solder cream as in Example 9 was printed by means of a0.15 mm thick metal mask to cover the first and second fuse bases 83Aand 83B, and then it was melted by heating to form 0.1 mm thick fuse F".The flux produced over the surface of the fuse F" was left withoutwashing. Porcelain enameled metallic substrates produced in such amanner were prepared in the number of 100.

The porcelain enameled metallic substrates underwent a test in whicheach substrate was heated at a rate of temperature rise of 10° C./minwhile it was inclined 45° to the horizontal plane with the first fusebase 83A placed below the second fuse base 83B. When the substrates wereheated to 183° C., the fuses F" partly began to melt, and all parts ofthe fuses F" became molten in five seconds from beginning of themelting. It was confirmed most of the molten fuses F" stayed on thefirst fuse bases 83A, and that any fuse were not left in the gap G,thereby positively breaking the circuits.

EXAMPLE 11

A 0.6 mm thick metal core 11 was used for each porcelain enameledmetallic substrate 3 having four fitting portions 71, and distal endportions 103 thereof were, as in FIG. 33, tapered so that a=0.25 mm andb=1.0 mm. Only one surface of the metal core 11 was coated with a 0.12mm thick porcelain enamel layer 13. The average thickness of the fittingportions 71 of the substrates at each of positions (1), (2), (3) and (4)in FIG. 33 is given in Table 9B, the positions (1), (2), (3) and (4)being 0.3 mm, 0.6 mm, 1.2 mm and 2.0 mm away from the distal end 105,respectively. Also given in Table 9B are the average thickness offitting portions 71 of ordinary porcelain enameled metallic substratesas shown in FIG. 37.

These two types of substrates having four fitting portions of FIG. 33and substrates having four ordinary fitting portions of FIG. 37 werecompared as to inserting force necessary to insert fitting portions intorespective metallic terminals of a feeder connector. The inserting forceis measured five times by means of a TCM-200 testing machine.

The results are given in Table 10. It was confirmed that the tapereddistal end portions reduced the inserting force by about 36%. Thus, thetapered distal end portions were capable of fitting into a feederconnector without providing any damage to both the conductor circuit andthe substrate. The tapered metal core of the fitting portions 71prevented the porcelain enamel layer 13, coated over it, from beingswelled.

                  TABLE 9B                                                        ______________________________________                                               Thickness of fit-      Thickness of fit-                               Position                                                                             ting portions (mm)                                                                          Position ting portions (mm)                              ______________________________________                                        (1)    0.600         (5)      0.84                                            (2)    0.765         (6)      0.87                                            (3)    0.835         (7)      0.83                                            (4)    0.820         (8)      0.82                                            ______________________________________                                    

                  TABLE 10                                                        ______________________________________                                        Inserting force                                                                      Terminals of FIG.      Terminals of FIG.                               No.    33 (kg · f)                                                                        No.      37 (kg · f)                            ______________________________________                                        1      8.8           1        11.2                                            2      9.2           2        13.0                                            3      7.2           3        14.5                                            4      6.8           4        10.9                                            5      8.5           5        13.8                                            Average                                                                              8.1           Average  12.7                                            ______________________________________                                    

EXAMPLE 12

Steel cores 11 having a thickness 0.6 mm were prepared each having fourterminal supporting portions having their distal end portions 103tapered as in FIG. 38. The steel core of each distal end portion 103 wastapered from a position, 1 mm away from a 0.25 mm thick distal end 105,toward the latter. The metal cores 11 were coated with a 150 μm thickcrystallized porcelain enamel layer. Two types of flat resistances wereprepared, one having conductor circuits 109 printed on one surface 104Aof each of the four terminal supporting portions 51 as in FIG. 38 andthe other on the opposite surfaces 104A and 104B as in FIG. 39. Ordinarytype flat resistances which had four terminal supporting portionswithout any tapered distal end portions as illustrated in FIG. 40 wereprepared. These three types of flat resistances were subjected to thesame test as in Example 11. The speed of inserting the fitting portionsof the terminal supporting portions was 20 mm/min.

The results are given in Table 11, from which it is clear that the flatresistances having fitting portions of FIG. 38 and fitting portions ofFIG. 39 reduced the inserting force to 61% and 53% of the fitting forcenecessary for the flat resistances with fitting portions of FIG. 40.

                  TABLE 11                                                        ______________________________________                                        Inserting force                                                                     Fitting portions                                                                           Fitting portions                                                                           Fitting portions                              No.   of FIG. 38 (kg · f)                                                               of FIG. 39 (kg · f)                                                               of FIG. 40 (kg · f)                  ______________________________________                                        1     8.3          7.0          11.2                                          2     8.9          7.1          13.0                                          3     7.3          7.0          14.5                                          4     7.1          6.2          10.9                                          5     7.2          6.0          13.8                                          Aver- 7.8          6.7          12.7                                          age                                                                           ______________________________________                                    

EXAMPLE 13

0.6 mm thick metal cores 11 each having four terminal supportingportions 71 were prepared with tapered distal end portions 103. Each ofthe distal end portions 103 was tapered thicknesswise from a position, 1mm away from a 0.25 mm thick distal end 105, toward the latter. Themetal cores 11 were each coated with a 130 μm thick crystallizedporcelain enamel layer 13. A printed pattern of a resistance circuit wasformed on each of the porcelain enameled metallic substrates andconductor circuits 109 were formed on the fitting portions 71 thereof asin FIG. 41. A colloidal graphite which was sold by Nippon AchisonKabushiki Kaisha, Japan, under a trademark Arodaccku G was splayed overthe opposite surfaces 104A and 104B of each fitting portion with athickness about 1-4 μm.

The substrates with lubricated fitting portions thus prepared underwentthe same test as in Example 12 and the results are indicated in Table12, from which it is noted that the substrates with the lubricatinglayer reduced the inserting force to 1/3 of that of the substrateshaving fitting portions of FIG. 40 without any lubricating layer,mentioned in Example 12. The substrates with lubricated fitting portionsof this example and the substrates having fitting portions shown in FIG.40, which had no lubricating layer, were measured in contact resistancebetween terminals. The former had an average contact resistance of 1.231mΩand the latter an average contact resistance of 1.209 mΩ. It was thusconfirmed that the rise in contact resistance due to the lubricatinglayer was negligibly small.

                  TABLE 12                                                        ______________________________________                                        Substrate with lubricated                                                                        Substrate (FIG. 40) with                                   fitting portions (Kg · f)                                                               no lubricating layer (Kg · f)                     ______________________________________                                        1     6.0              11.2                                                   2     4.5              13.0                                                   3     3.0              14.5                                                   4     4.8              10.9                                                   5     3.9              13.8                                                   Aver- 4.4              12.7                                                   age                                                                           ______________________________________                                    

What is claimed is:
 1. A flat resistance comprising:a porcelain enameledmetallic substrate including a flat head portion, having an edge, andparallel terminal supporting the head portion and the terminalsupporting portions forming one surface; a resistance circuit printed onthe one surface of the head portion, the resistance circuit including aplurality of electrically interconnected resistances, at least one ofthe resistances having a first portion and a second portion, said firstportion and said second portion each having a fuse connecting end, saidfuse connecting end defining a gap between the first portion and thesecond portion, at lest one of the fuse connecting ends comprising fuseattracting means; a temperature fuse interposed in the resistancecircuit and adapted to melt to break the resistance circuit when theporcelain enameled metallic substrate substantially reaches apermissible maximum temperature at a portion thereof, the temperaturefuse being provided in the vicinity of the portion of the substrate, thetemperature fuse being electrically coupled to the fuse connecting endsof the first portion and the second portion, the fuse attracting meansattracting the temperature fuse when the temperature fuse is in a moltenstate when the substrate reaches the permissible maximum temperature;and terminal means, printed on the one surface of both the head portionand the terminal supporting portions, the terminal means includingterminals each connected to one end of a corresponding one of theresistances.
 2. A flat resistance as recited in claim 1, wherein eachterminal includes a fitting portion to fit to a feeder connector; andwherein the resistance circuit comprises an electrically conductivecircuit printed on the one surface of the metallic substrate to beinterposed between the fitting portion of each terminal and thecorresponding resistance, the conductive circuit having a sheetresistance smaller than the corresponding resistance for effectivelypreventing overheating of the fitting portion of the terminal.
 3. A flatresistance as recited in claim 1, wherein each terminal includes afitting portion to fit to a feeder connector; and wherein the resistancecircuit comprises an electrically conductive circuit printed on the onesurface of the metallic substrate to be deposited on the correspondingresistance for electrical connection to the fitting portion of eachterminal, the conductive circuit having a sheet resistance smaller thanthe corresponding resistance for effectively preventing overheating ofthe fitting portion of the terminal.
 4. A flat resistance as recited inclaim 1, further comprising an attaching means for attaching thesubstrate to a casing of an air conditioner, and locking means fordetachably locking the substrate to a supporting frame; wherein thesupporting frame comprises a head and a socket integrally formed withthe supporting frame to extend downwards from the head, the sockethaving an opening downwards to receive a feeder coupler for connectingto the terminals, wherein the terminal supporting portions pass throughthe head into the socket, and wherein the locking means comprises afirst locking member, mounted to the substrate, and a second lockingmember, mounted to the supporting frame, the first and second lockingmembers being adapted to engage with each other when the terminalsupporting portions of the substrate are fitted into the socket throughthe head.
 5. A flat resistance as recited in claim 1, wherein thetemperature fuse comprises a thin film fuse having a thicknessapproximately 0.1 mm to 1.0 mm.
 6. A flat resistance as recited in claim5, wherein the temperature fuse is interposed in a resistance having abroken portion, the broken portion including a pair of fuse connectingends defining a gap therebetween, wherein the temperature fuse iselectrically connected at opposite ends thereof to corresponding fuseconnecting ends, and wherein the thickness, the length and the gap ofeach fuse connecting end are more than two times as large as thethickness of the temperature fuse.
 7. A flat resistance as recited inclaim 6, wherein the substrate is adapted for inclined arrangement to ahorizontal plane, wherein the fuse connecting ends include an upper fuseconnecting end and a lower fuse connecting end larger in both width andarea than the upper fuse connecting end.
 8. A flat resistance as recitedin claim 1, wherein the temperature fuse has a surface layer made of aflux.
 9. A flat resistance as recited in claim 1, wherein thetemperature fuse is interposed in one of the resistances having a brokenportion, the broken portion including a pair of fuse connecting endsdefining a gap therebetween, wherein the temperature fuse iselectrically connected at opposite ends thereof to corresponding fuseconnecting ends, and wherein at least one of the fuse connecting endscomprises fuse attracting means, electrically connected to acorresponding end of the temperature fuse, for attracting a temperaturefuse in a molten state when the substrate reaches the permissiblemaximum temperature.
 10. A flat resistance as recited in claim 9,wherein the attracting means is good in wettability to the moltentemperature fuse as compared to the one resistance.
 11. A flatresistance as recited in claim 9, wherein the at least one fuseconnecting end comprises an irregular upper surface as the attractingmeans, the irregular upper surface adapted to attract the moltentemperature fuse by means of capillary phenomenon.
 12. A flat resistanceas recited in claim 1, wherein each of the terminal supporting portionscomprises a metal core having a proximal portion and a distal end, themetal core of the terminal supporting portion being tapered toward thedistal end for facilitating fitting of a feeder coupler around thecorresponding terminal.
 13. A flat resistance as recited in claim 12,wherein the metal core of each terminal supporting portion is tapered adistance approximately 1.0 to 3.0 mm from the proximal portion of themetal core to the distal end so that the distal end has a thicknessranging approximately from 0.10 to 0.30 mm and the proximal portion hasa thickness ranging approximately from 0.5 to 0.7 mm.
 14. A flatresistance as recited in claim 12, wherein each terminal extends in thevicinity of the distal end of the corresponding terminal supportingportion for reducing friction to the feeder coupler.
 15. A flatresistance as recited in claim 14, wherein each terminal supportingportion has another surface opposite to the one surface, and wherein theterminal supporting portion has an electrically conductive circuitprinted on the another surface thereof to extend to the vicinity of thedistal end for reducing friction to the feeder coupler.
 16. A flatresistance as recited in claim 14, wherein each terminal supportingportion comprises an electrically conductive lubricating layer coatedover the corresponding terminal to the distal end thereof for reducingfriction to the feeder coupler.
 17. A flat resistance comprising:aporcelain enameled metallic substrate including a flat head portion,having an edge, and at least three parallel terminal supporting portionsprojecting outwardly from the edge of the head portion, the head portionand the terminal supporting portions forming one surface; a resistancecircuit printed on the one surface of the head portion, the resistancecircuit including at least three resistances electrically connected inseries-parallel fashion, each of the resistances having differentresistance values, at least one of the resistances having a firstportion and a second portion; a temperature fuse interposed in theresistance circuit and adapted to melt to break the resistance circuitwhen the porcelain enameled metallic substrate substantially reaches apermissible maximum temperature at a portion thereof, t he temperaturefuse being provided in the vicinity of the portion of the substrate, thetemperature fuse being electrically coupled to the first portion and thesecond portion;; and terminal means, printed on the one surface of boththe head portion and the terminal supporting portions, the terminalmeans including at least three electrically conductive terminals eachconnected to both one end of the one of said at least three resistancesand one of said terminal supporting positions, said electricallyconductive terminals having a sheet resistance smaller than theresistance to which it is connected.
 18. A flat resistance as in claim17, wherein said first portion and said second portion each have a fuseconnecting end, such fuse connecting means defining a gap between thefirst portion and the second portion, at least one the fuse connectingends comprising fuse attracting means, the temperature fuse beingelectrically coupled to said fuse connecting ends and the fuseattracting means attracting the temperature fuse when the temperaturefuse is in a molten state when the substrate reaches the permissiblemaximum temperature.
 19. A flat resistance as in claim 18, furthercomprising an attaching means for attaching the substrate to a casing ofan air conditioner, and locking means for detachably locking thesubstrate to a supporting frame, wherein the supporting frame comprisesa head and a socket integrally formed with the supporting frame toextend downward from the head, the socket having an opening downward toreceive a feeder coupler for connecting to the terminals, wherein theterminal supporting portions pass through the head into the socket, andwherein the locking mean comprises a first locking member, mounted tothe substrate, and a second locking member, mounted to the supportingframe, the first and the second locking members being adapted to engagewith each other when the terminal supporting portions of the substrateare fitted into the socket through the head.